#include<iostream>
using namespace std;
int main() {
	int a, b, c;
	cin >> a >> b >> c;
	int d = 0;
	for (int i = 0; i < c; i++) {
		int k;
		cin >> k;
		d += k;
	}
	if (a*b < d) {
		cout << -1 << endl;
	}
	else {
		cout << a * b - d << endl;
	}
}